The present invention relates in general to methods and apparata for inductively heating and quench hardening a crankshaft. More specifically, the present invention relates to inductively heating and quench hardening a crankshaft, which may be oriented either horizontally or vertically, wherein the induction coil assembly (or assemblies) do not contact the surfaces of the crankshaft which are to be induction hardened. Computer controlled servomotors and X and Y drive systems are used to position and move the induction coil assembly relative to a crankpin portion of the crankshaft as the crankshaft rotates at a predetermined RPM. The travel of the induction coil assembly is based upon mathematical formulae and the crankshaft geometry, including crankshaft dimensions and the particular location of the crankpin portion to be induction hardened relative to the longitudinal axis of the crankshaft.
An automotive crankshaft is made up of a series of crankpins, one for each cylinder, in the case of in-line engines, or one for each pair of cylinders, in the case of V-type engines. The function of the crankshaft is to convert the reciprocating motion of the piston and its connecting rod into rotating motion. The throw of the crankshaft is equal to the stroke of the engine. The crankshaft needs to be properly balanced in order to eliminate centrifugal forces and accordingly the crankshaft is counterbalanced by weights placed opposite to the corresponding crankpins or just "pins". Each pin is received within one end of a corresponding connecting rod whose opposite end is pinned to a piston. Crankshafts are also configured with axial bearing surfaces which are designed for receipt by the main bearings. A six cylinder in-line crankshaft would typically have seven main bearings.
Due to the load and wear on the pins and on the bearing surfaces, the hardening of these portions of the crankshaft is important. One approach to this task is to inductively heat and then quench harden these critical surfaces. Traditionally the approach which has been followed is to place the crankshaft in a horizontal orientation and as the crankshaft reaches a substantially elevated temperature due to the induction heating, a support member is moved into position in order to support the crankshaft and keep it from sagging. This traditional approach also involves the induction coil and/or some portion of the induction coil assembly contacting and in fact actually riding on the surfaces which are to be inductively heated and quench hardened. This metal-to-metal contact accelerates the wear on the coil assembly, necessitating that the coil assembly be replaced periodically. The need to replace the induction coil assembly represents not only an added cost factor but also down time to the induction hardening equipment.
By orienting the crankshaft horizontally, the contact by the induction coil assembly on the critical surfaces of the crankshaft is actually encouraged due to the convenience of letting the induction coil assembly "ride" on the pins and bearing surfaces as the crankshaft is rotated between centers. This traditional approach of having the induction coil assembly function like a follower does not require any separate drive system for the induction coil assembly since the critical surfaces are in contact with the coil assembly. However, direct contact between the coil assembly and the portion of the crankshaft to be induction hardened is seen as a substantial disadvantage, not only due to wear of the induction coil assembly and the horizontal mounting of the crankshaft, but for the additional reasons which are set forth below.
When the induction coil assembly contacts the pins and/or bearing surfaces, it is difficult to identify the wear condition of the coil assembly. By riding directly on the crankshaft surfaces, the contacting surface of the induction coil assembly is effectively hidden from view, thereby making it difficult to assess the level or degree of wear on the coil assembly. This in turn means that the induction coil assembly can be run too long and reach a point at which it arcs out and this typically ruins the part and ruins or damages the coil assembly. Contact between the coil assembly and the crankshaft often results in marring or galling of the crankshaft surface and this requires extra grind stock which can then be machined away in order to grind out the surface imperfections. An extended post-hardening step is then required.
It would be a substantial improvement to the present methods and apparata for induction hardening crankshafts if an apparatus could be provided whereby the induction coil assembly does not have to contact the pins and bearing surfaces. Such an apparatus would significantly improve coil assembly life. It is also felt that being able to orient the crankshaft vertically would be advantageous. While the prior art does not envision any suitable solution to the problems which have been identified, the present invention provides an improved method and apparatus which achieves both improvements. According to the present invention, the crankshaft which is to be induction hardened can be vertically oriented, even though the present invention still works quite well if the crankshaft is oriented horizontally. Further, one induction coil assembly is provided for the crankshaft pins and is located and operated at a first workstation. Either a separate induction coil assembly or a series of assemblies are provided for the bearing surfaces and are located and operated at a second workstation. These coil assemblies are designed such that there is no contact with the crankshaft surfaces which the coil assemblies are to induction harden. This improves the coil assembly life. According to the present invention, the dimensions and geometry of the crankshaft are used to define the path or orbit of each pin and the tracking path for each induction coil assembly is computed and programmed into suitable drive systems which control the travel of each coil assembly. While the bearing surfaces also have an orbit, these orbits are concentric with the axis of rotation of the crankshaft. Accordingly the coil assembly (or assemblies) used for these bearing surfaces does not have to travel in a matching orbit, but instead is stationary. Alternate embodiments of the present invention provide design variations to account for the presence of counterweights or for the presence of any other factor which could affect the balance of mass (heat balance) adjacent the pins of the crankshaft.
While there are other designs which suggest a vertical orientation for the workpiece, these other designs are limited to camshafts, not crankshafts. There are numerous differences between these two types of drive components, several of which suggest that technology directed to camshafts has very little relevancy to the present invention and the issues which are addressed and solved by the present invention.
For example, the individual cams of a camshaft are axially mounted and the protruding portion of the cam geometry is dimensionally fairly minor. There simply is not the off-axis dimensional shift for cams the way there is for the pins of a crankshaft. This results in a pin orbit of substantial size and travel relative to whatever cam orbit might be present. In turn, this results in substantially different challenges and problems for the design of a suitable induction coil tracking apparatus, with the crankshaft presenting the more challenging design task.
With regard to the comparison between a crankshaft and a camshaft, the profile of a crankpin is symmetrical and requires a uniform case depth. A cam of a camshaft is not symmetrical and does not require a uniform case depth. Accordingly, the induction coil assembly does not have to follow a cam and the cam can be induction hardened without having to move the coil assembly in a matching orbit. The desired case depth patterns for the cams can be achieved without displacement of the induction coil assembly. The lower loads placed on a cam mean that the required hardness depth can be less than that of a crankshaft pin, causing less demanding induction hardening. While the present invention can be used for a camshaft, there is no reason to do so.
Another feature addressed by the present invention is the arrangement of the handling equipment and the cooperating workstations. In order to provide handling efficiencies, the present invention is configured with multiple workstations for the loading, induction hardening, and unloading of the workpiece in sequential action.
One workstation is configured for induction hardening of the crankshaft pins. Another workstation is configured for induction hardening of the bearing surfaces. These two workstations may be arranged in either order since the pins and bearing surfaces can be induction hardened in any order. Since the bearing surfaces are coaxial with the centers supporting the crankshaft, the induction coil assembly for the bearing surfaces operates in an orbit which is coincident with the axis of rotation (longitudinal axis of the crankshaft). In contrast, the pins which are sequentially induction hardened, typically one pin at a time, are not located on-axis and have a different circumferential location, one pin to the next, relative to the position of the crankshaft.
While induction hardening of crankshafts is known and while the vertical orientation of camshafts is known, the present invention remains novel and unobvious. The combination of structural features of the present invention provides significant advantages to what presently exists and the long felt and heretofore unsatisfied need for the present invention validates its novel and unobvious advance in the art.